1. Field of the Invention
The present invention relates to a laminate, a ceramic substrate and a method for making the ceramic substrate.
2. Description of the Related Art
In conventional circuit boards including ceramic insulating substrates composed of, e.g., alumina, and wiring layers, a material containing molybdenum, tungsten, or other elements that can withstand high temperatures, as a main component has been used in the wiring layers since the insulating substrate is fired at a temperature of about 1500° C. or more in order to fire the insulating substrate and the wiring layers simultaneously. However, rapid progress in information and communication technology in recent years has brought about an increase in both speed and size of semiconductor elements and other associated components. Accordingly, circuit boards equipped with such elements are required to achieve reduction of signal transmission loss by decreasing the resistance of the wiring layers.
In this respect, a circuit board including a wiring layer mainly composed of silver, copper, or gold with a low conductor resistance and an insulating substrate composed of a low-temperature-sintering ceramic that can be densified at a temperature 1000° C. or less, i.e., the temperature at which the wiring layer does not melt, has been proposed.
The transmission loss of the wiring layer is mainly affected by the conductor resistance for DC signals and low-frequency signals. In contrast, when signals transmitted in the wiring layer have high frequency exceeding several hundred megahertz, the skin effect is the highly influential factor. Skin effect is concentration of electric current in a narrow region about several micrometers in thickness from the interface between the conductor and the insulating substrate or from the surface of the conductor. Thus, when the conductor surface or the interface is rough, the conductor resistance substantially increases at high frequencies. That is, the effective conductivity is decreased. As a result, signal transmission loss is increased.
Therefore, smoothing of the conductor surface or interface is required to increase the effective conductivity and decrease the transmission loss.
A circuit board is usually used with semiconductor elements and the like mounted thereon or by mounting the circuit board on an external circuit board or the like. In order to prevent breaking by thermal stresses caused by the difference in thermal expansion coefficient between the circuit board and the semiconductor element or the external circuit board and by mechanical stresses caused by dropping, vibrations, and the like, the transverse rupture strength of the circuit board must be high.
A glass ceramic that contains a glass powder composed of SiO2, Al2O3, CaO, MgO, and B2O3, an Al2O3 powder, and a celsian (BaAl2Si2O8) powder has been proposed as a low-temperature-sintering ceramic having high transverse rupture strength. By using the glass ceramic in the insulating substrate, a circuit board with a low residual carbon content can be obtained even when copper used in the wiring layers is fired in a non-oxidative atmosphere without degrading the ability of removing the organic binder.
Another proposed low-temperature-sintering ceramic with high transverse rupture strength is a glass ceramic containing at least one of a gahnite crystal phase and a spinel crystal phase, a celsian crystal phase including acicular crystals having an aspect ratio of 3 or more, and at least one crystal phase selected from the group consisting of AlN, Si3N4, SiC, Al2O3, ZrO2, Al2O3.2SiO2, and Mg2SiO4.
However the two proposed circuit boards have a problem in that although the mount reliability can be improved by increasing the transverse rupture strength of the circuit board, firing shrinkage in the X-Y direction (a direction parallel to a main surface of a circuit substrate) caused by firing is large. Accordingly, the dimensional accuracy of the electrode mounted on the circuit board is degraded in the X-Y direction, and it has been difficult to mount a semiconductor element or connector with narrow-pitch terminals, or a small chip component.
On the other hand, demand for smaller, thinner circuit boards is increasing in recent years, and miniaturization of wiring layer patterns is in progress. Thus, in order to increase the dimensional accuracy of the wiring layer in the X-Y direction, there has been proposed a circuit board with a small firing shrinkage in the X-Y direction and a small variation in the firing shrinkage in the X-Y direction, the circuit board being made by preparing and laminating green sheets that undergo firing shrinkage at a low temperature and green sheets that undergo firing shrinkage at a high temperature form a laminate and firing the laminate.
Examples of the green sheets that undergo firing shrinkage at a low temperature include glass ceramic sheets includes a glass containing 10 to 40 percent by mass of SiO2, 35 to 60 percent by mass of MgO, and 10 to 30 percent by mass of B2O3. Examples of the green sheets that undergo firing shrinkage at a high temperature include glass ceramic sheets including a glass containing 20 to 50 percent by mass of SiO2, 3 to 25 percent by mass of MgO, and 0 to 55 percent by mass of at least one selected from the group consisting of B2O3, CaO, Al2O3, SrO, ZnO, TiO2, Na2O, BaO, SnO2, P2O3, ZrO2, and Li2O.
However, when the above-described green sheet for a circuit board with increased strength is used as a green sheet 2 that undergoes firing shrinkage at a high temperature and this green sheet 2 is used in combination with a green sheet 1 that undergoes firing shrinkage at a low temperature and when these two types of green sheets are laminated and fired in attempt to reduce the variation in firing shrinkage in the X-Y direction, the green sheet 2 rarely undergoes firing shrinkage in the X-Y direction but only in the Z direction. Thus, voids are formed by firing. For example, a thin green sheet with an inter wire layer distance of about 25 μm suffers from degradation in isolated resistance due to the voids generated.